Neptune
is like Uranus in many ways, but has its own unique
features.
Because of Pluto's highly
elliptical orbit, it
is currently the most distant planet from the Sun, at a separation of
about 30 Astronomical Units. The full disk of Neptune in shown in the
adjacent image
(Ref).

Neptune has been particularly challenging
to study from the ground because its disk is small and badly blurred by
the Earth's atmosphere at that distance.
In spite of this, ground-based astronomers had
learned a great deal about this planet since its position was first
predicted by Adams and Leverrier in 1845.
However, our most detailed information about Neptune has come from the Voyager
2 flyby in 1989.